Why: newest DFF supports Firefox 48+; newest Tor releases are still based on Firefox 45.

Solution: please use older version of the plugin when working with Tor: https://addons.mozilla.org/en-US/firefox/addon/dummy-form-filler/versions/?page=1#version-0.13.2
0.13.2 is still very useful as it holds the most important features.

Solution 2: patience. The next major update of Tor (somewhere in 2017) will be based on Firefox 52 and this will end compatibility problems.